|Publication number||US20060103365 A1|
|Application number||US 10/989,337|
|Publication date||May 18, 2006|
|Filing date||Nov 17, 2004|
|Priority date||Nov 17, 2004|
|Publication number||10989337, 989337, US 2006/0103365 A1, US 2006/103365 A1, US 20060103365 A1, US 20060103365A1, US 2006103365 A1, US 2006103365A1, US-A1-20060103365, US-A1-2006103365, US2006/0103365A1, US2006/103365A1, US20060103365 A1, US20060103365A1, US2006103365 A1, US2006103365A1|
|Original Assignee||Compulite Systems (2000) Ltd.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (20), Classifications (4), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to a switching power converter and, more particularly, to a method and circuitry for converting an alternating-current (AC) power line voltage into a lower AC voltage while drawing from the line a current of proportionally lower amplitude.
The prior art includes several types of converters, which are widely used for DC(direct current)-to-DC, DC-to-AC, AC-to-DC and AC-to-AC power conversion. In some applications the purpose of the converter is to provide a regulated output voltage. In other applications, the purpose of the power conversion schemes is attenuating the voltage of an AC source, such as a power line, while maintaining the waveform of the line voltage. For example, in a power converter known in the art as an AC Chopper, the role of the converter is to supply to a load an AC voltage lower than the voltage of the AC source. This is accomplished by introducing an AC (bidirectional) switch between the line and load and operating the switch at a given duty cycle, D, where D is the fraction of the time that the switch is in a conductive, or “on” state. Such a chopper feeds the load ‘slices’ of the source voltage, resulting in a lower average power delivered to the load, relative to the load power when the load is connected directly to the source. If the chopping is carried out at high frequency, as compared to the frequency of the source (typically, the chopping frequency is above 10 kHz in 50 Hz or 60 Hz power line applications), and a filter is introduced between the AC switch and the load, then the waveform of the voltage applied to the load substantially resembles the waveform of the source voltage, although attenuated. The output voltage is then substantially equal to the input voltage times the duty cycle, D (neglecting losses in the converter).
If the load is linear, the load current is equal to the load voltage divided by the impedance of the load. This current, drawn from the source, is chopped by the AC switch of the chopper. Consequently the source current is the output current times the duty cycle, D and, with appropriate filtering to remove high-frequency components, the waveform of the source current substantially resembles the waveform of the load current, although attenuated. If the load is linear and the source voltage is sinusoidal then the source current is also substantially sinusoidal, i.e., of small harmonic content and with low harmonic distortion. This is of special practical importance considering the harmful effects that high levels of current harmonics have on power lines. Among these harmful effects are reduced efficiency of power transmission, possible interference to other devices connected to the power line and distortion of the waveform of the line voltage. In light of these harmful effects, many regulatory authorities have adopted, or are in the process of adopting, voluntary and mandatory standards and statutes which set limits on permissible line current harmonics injected by any given electrical equipment powered by the AC mains, in order to maintain relatively high quality of power.
Considering the above, AC choppers are good solutions for cases in which there is a need to control the AC power of a linear load while maintaining a sinusoidal line current, that is, of low harmonic content. Incandescent lamp dimmers fall into this application category. For any given power level, an incandescent lamp practically represents a resistive load and hence, when driven by an AC chopper, draws from the power line a substantially sinusoidal current. AC choppers are therefore a preferred approach for realizing incandescent lamp dimmers. An earlier approach to dimmer design that was based on phase control, i.e. a low frequency chopper typically realized with SCR (Silicon Controlled Rectifier, or Thyristor) switches, has many deficiencies, including the fact that it injects a very high level of current harmonics into the line. Among the disadvantages of the low-frequency chopper are the mechanical stress on lamp filaments when fed by low-frequency high-current pulses, and high peak current in the line that may blow fuses or trip circuit breakers. It is thus evident that the high-frequency chopper is a preferred design solution for line-fed controlled-voltage AC sources in general, and for incandescent lamp dimmers in particular.
Practical design of high-frequency (HF) AC choppers requires the incorporation of an additional switch, aside from the series chopping AC switch. This is because the output filter typically includes a series inductor, and when the current through the inductor is interrupted when the serially connected AC switch is turned off an alternative conduction path must be provided for the current. Otherwise, very large and damaging voltage spikes develop. Many solutions have been proposed in the past to overcome this problem. Among these solutions is the incorporation of another AC switch between the input terminals of the output filter, i.e., connected as a shunt, in the manner of the connection in
However, prior-art HF AC chopper systems, such as the teaching of U.S. Pat. No. 5,500,575 and other publications, suffer from a basic problem that leads to the generation of high-voltage spikes and power losses, and hence, lower efficiency. The problem stems from the need to leave a “deadtime” during which neither the series AC switch nor the AC switch (or diode-switch assembly) connected at the input terminals of the output filter are conductive. This is necessary to avoid a so-called “overlap” or “shoot-through” situation in which the input source is shorted by the two conductive switches that are effectively serially connected across the source. During this deadtime the current of the output filter inductor is interrupted and a potentially damaging high-voltage spike develops. This voltage might exceed the breakdown voltages of the active and passive elements of the circuit and cause irreversible damage to the system. To overcome this problem, Ahmed et al. teaches the placement of a capacitor at the input to the output filter. This “snubber” capacitor provides a path for the filter inductor current, and, if large enough, prevents the development of large voltage spikes. However, this capacitor causes large current spikes and losses in the series AC switch. This happens when the switch is turned on and is effectively connecting the AC source to the capacitor. The voltage difference between the input voltage and the capacitor voltage causes high charging current, limited only by the parasitic resistances of the line and switch.
It is thus evident that the prior art has failed to provide a remedy for the problem of the deadtime in HF AC choppers.
Because the issue of current harmonic injection to the power line is of great importance, as is evident from the standards that are enforced by many regulatory authorities, close control of the input current waveform is a desirable feature of an AC chopper. Prior art teachings do not adequately address this issue but, rather, assume that when the HF AC chopper operates in “open loop” (with respect to the input current), the input current is automatically of low harmonic content. This is correct if two conditions are fulfilled: first, that the load is linear and, second, that the input filter and the output filter both operate with the inductor current in Continuous Conduction Mode (CCM). That is, that the currents in the filters never reach zero. As is very well known in the art, any filter leaves the CCM and enters the Discontinuous Conduction Mode (DCM) if the load resistance is increased sufficiently. The boundary between CCM and DCM depends upon the value of the inductance of the filters. Thus, any HF AC chopper tends to enter DCM for low-power loads. This causes a distortion of the input current and prior-art solutions fail to provide a remedy for this problem, too.
In some applications, the waveform of the output voltage is an important consideration. For example, because harmonics are known to cause losses in motors, it is desirable to feed motors with a pure sinusoidal waveform. However, in prior art solutions the waveform of the output voltage is a replica of the waveform of the input voltage. Thus, if the input voltage waveform is distorted, the output waveform also is distorted.
It is thus evident that prior-art teachings fail to provide solutions to many problems associated with the design of a HF AC chopper.
Therefore, it is desirable to have a HF AC chopper, for incandescent lamp dimmers and other applications, in which the deadtime is eliminated, thus avoiding the problem of high-voltage spikes and the need for snubber capacitors, both of which cause electrical noise and power losses.
It is further desirable that a HF AC chopper be able to feed not only resistive loads but reactive loads as well, in which there is a phase difference between the load voltage and the load current and energy is fed back to the source during part of the cycle, while still operating the chopper without a deadtime interval.
It is also desirable for the HF AC chopper to include inrush-current control and over-current protection to increase the robustness and reliability of the chopper and protect devices connected to the chopper.
It is also desirable that a HF AC chopper be able to operate in closed-loop configurations for cases that require precise control of the output voltage.
It is further desirable that a HF AC chopper be able to operate in closed-loop configurations for cases that require precise control of the input current.
It is further desirable that a HF AC chopper be able to operate in closed-loop configurations for loads that require precise control of output current, such as discharge lamps and certain types of motors.
There is thus a widely recognized need for, and it would be highly advantageous to have, an economical, rugged, robust and compact AC chopper capable of handling reactive loads efficiently and drawing from the power line a current having minimal harmonic content, without a need for deadtime, without a need for snubber components associated with the chopping switches, and capable of including such features as inrush-current control, over-current protection, and closed-loop control of output voltage, input current, or output current, including when the chopper filters operate in DCM.
As used herein, unless otherwise specified, the term “valve” refers to a device for controlling electric current. Valves may include, but are not limited to, vacuum tubes, gas-filled tubes, vapor-filled tubes, diodes, switches, Insulated-Gate Bipolar Transistors (IGBT), Metal-Oxide Semiconductor Field-Effect Transistors (MOSFET) and Bipolar Junction Transistors (BJT) and combinations thereof. Valves may serve functions including, but not limited to, rectification, switching, and amplification. The particular type of valve intended is clear from context. Although this usage of the term “valve” is inspired by the British usage of that term to describe vacuum tubes, the usage here is, per the above definition, different and broader.
As used herein, unless otherwise specified, the term “power line” refers to a source of electrical power, including, but not limited to, commercial power mains, generators, inverters, and power systems on board automobiles, trucks, ships, submarines, aircraft, spacecraft, and other vehicles.
As used herein, unless otherwise specified, a waveform of a parameter is a set of values of that parameter as a function of time. Parameters that may have waveforms include, but are not limited to, voltage and current.
As used herein, unless otherwise specified, a voltage and current are said to be “in phase” when the current is flowing in the direction that current would flow when the voltage is loaded by a resistor.
As used herein, unless otherwise specified, a voltage and current are said to be “out of phase” when the current is flowing in a direction opposite to the direction that current would flow when the voltage is loaded by a resistor. Examples of out of phase currents include the charging of a battery and the return of energy to an AC power line by a reactive load during a portion of the AC cycle.
It is an object of the present invention to provide HF AC chopper circuitry that does not rely on deadtime and always provides a current path for the current of the inductor of the output filter, simplifying construction and increasing the reliability, efficiency and flexibility of the chopper for a variety of applications including lamp dimming and motor control.
It is another object of the present invention to eliminate the need for snubber capacitors in HF AC choppers to avoid large charging currents.
It is yet another object of the present invention to provide economical digital circuitry for improving the performance and ruggedness of HF AC choppers.
It is yet another objective of the present invention to provide HF AC chopper circuitry that integrates other functions within the chopper, such as inrush-current control, over-current protection and output-voltage control to increase the reliability, economy and range of application of AC choppers.
It is also an object of the present invention to provide a HF AC chopper that provides tight control of the input current even if the filters of the chopper enter DCM.
It is yet another object of the present invention to provide a HF AC chopper that provides tight control of the output voltage even if the filters of the chopper enter DCM.
The present invention is directed to a method for converting the voltage of an AC source into a voltage of lower amplitude of the same waveform as the input voltage.
It is common practice for single-phase AC power lines to include a phase conductor and a neutral conductor. The neutral conductor is substantially at ground potential, while the potential of the phase conductor varies sinusoidally with respect to the potential of the neutral conductor. The phase conductor can be referred to by such terms as “hot”, “high”, or, simply, “phase” and the neutral conductor can be referred to by such terms as “low”, or, simply, “neutral”. The present invention is described herein with respect to this type of power line. However, other types of single-phase power lines, including power lines wherein both conductors are permitted to “float” with respect to ground potential, are possible. It will be readily apparent to those skilled in the art that the present invention is also applicable to such other types of power lines, and the use of such other types of power lines is within the scope of the present invention.
The present invention accomplishes voltage reduction by turning on and off a set of switches which connect an input terminal of an output filter inductor alternately to the phase conductor or to the neutral conductor of the input source, either directly or via diodes. The present invention makes optimal use of diode-switch combinations such that a conductive path is always provided for the inductor current, either to the phase side or to the neutral side of the input power line while, at the same time, avoiding a short circuit across the input power line. The present invention controls the output voltage of the AC chopper by a including a combination of analog circuitry and digital circuitry that drives the diode-switch assemblies in a manner that maintains the desired output voltage by duty-cycle control. The present invention also provides output voltage control that can generate a substantially pure sinusoidal output voltage even if the input voltage waveform is distorted. Furthermore, the present invention includes other important features, such as over-current protection and inrush-current control without adding considerable complexity and/or cost to the controller circuitry, while providing, at all times, a path for the output filter inductor current, thus avoiding dangerous voltage spikes.
Accordingly, the present invention is characterized by providing an inherent voltage clamp that operates as a lossless snubber.
The present invention discloses a HF AC chopper apparatus which has improved reliability, higher efficiency, and programmability features, the chopper including at least an output filter inductor and two controllable diode-switch assemblies, one diode-switch assembly connected between the phase side of the input source and the input terminal of the output filter inductor, and the other diode-switch assembly connected between the same inductor terminal and the neutral side of the input source.
Preferably, a switching power converter according to the present invention also includes:
inductor-current sampling circuitry, for sampling the instantaneous value of the main inductor current;
voltage sampling circuitry, for sampling the instantaneous polarity of the input voltage; and
a combination of analog circuitry and digital circuitry that is supplied with a reference signal, a signal indicative of the polarity of the inductor current, and a signal indicative of the polarity of the input voltage, the circuitry operative to control the switching of the diode-switch assemblies and thereby, to cause the output voltage of the AC chopper to follow the shape on the input voltage with a magnitude that is proportional to the reference signal, but limit the output current during inrush and overload conditions.
Optionally, the above-mentioned analog and digital circuitry is supplied with a signal proportional to the inductor current and a signal proportional to the input voltage. The use of signals merely indicating the polarities of the inductor current and the input voltage is sufficient for open-loop operation. The use a signal proportional to the inductor current or a signal proportional to the input voltage, or both, is preferable for closed-loop operation.
The combination of analog circuitry and digital circuitry preferably further includes:
Comparators operative to produce binary output signals indicating the polarity of the input voltage and the polarity of the inductor current, and an over-current signal that is active when the inductor current exceeds a predetermined level;
a time-base for determining the switching period;
a pulse width modulator (PWM) for controlling the duty cycle; and
logic circuitry to process the above-mentioned signals and produce drive signals for the switches.
According to the present invention there is provided a power converter system connecting an AC power line to a load, including: (a) a series valve in series with the load; (b) a shunt valve in parallel with the load; (c) an inductor in series with the load; and (d) a mechanism for configuring the valves so that the power converter operates as a buck converter while the inductor carries current in phase with the power line and as a boost converter while the inductor carries current out of phase with the power line.
Preferably, in the system, the mechanism includes: (i) a control signal, and the mechanism is operative to operate a valve according to a duty cycle ratio selected so as to cause a voltage across the load to be substantially in proportion to the control signal.
Alternatively, in the system, the mechanism includes: (i) a control signal; and (ii) a feedback sensor, and the feedback sensor is operative to sense, for use as a feedback signal, a parameter of the system, and the mechanism is operative to compare the feedback signal with the control signal and to operate a valve according to a duty cycle ratio selected so as to cause the feedback signal to be in proportion to the control signal.
Preferably, in the alternative system, the feedback signal is selected from the group consisting of an output voltage, an output current, an input current, an inductor current, a load power dissipation, a load temperature, a position, a speed, an acceleration, a force, a torque and a light intensity.
Preferably, the system further includes: (e) a mechanism for sensing an output current, and a valve is operated according to a duty cycle ratio selected so as to restrict the output current to be within a predetermined limit profile.
Preferably, the system includes: (e) a mechanism for sensing an output current, and, if the output current exceeds a predetermined limit profile, the valves are operated in a manner selected to interrupt the output current while providing a path for dissipation of a current flowing through the inductor.
Preferably, in the system, at least one valve includes a diode-switch assembly.
Preferably, in the system, at least one diode-switch assembly includes: (i) a first diode; (ii) a second diode; (iii) a first switch; and (iv) a second switch, and the first diode is serially connected to the second diode such that the first diode permits flow of current in a direction opposed by the second diode, and wherein the first switch is responsive to a first switch control signal and is connected in parallel with the first diode, and wherein the second switch is responsive to a second switch control signal and is connected in parallel with the first diode.
Alternatively, in the system, at least one diode-switch assembly includes: (i) a first diode; (ii) a second diode; (iii) a first switch; and (iv) a second switch, and the first diode is serially connected to the first switch, the first switch being responsive to a first switch control signal, and wherein the second diode is serially connected to the second switch, the second switch being responsive to a second switch control signal, the serially connected first diode and first switch being connected in parallel with the serially connected second diode and second switch, such that the first diode permits flow of current in a direction opposite to a direction the second diode permits flow of current.
Preferably, in the system, the mechanism includes: (i) a sawtooth waveform generator; (ii) a control signal; and (iii) a comparator, and the comparator is operative to compare an output of the sawtooth waveform generator with the control signal and to produce an output signal having a duty cycle ratio substantially in proportion to the control signal and operative to control a valve.
Preferably, in the system, the mechanism is operative, upon change of polarity of a power line voltage, to operate the valves in a manner that prevents shorting of the power line and provides a conductive path for current in the inductor.
Preferably, in the system, the mechanism is operative, upon change of polarity of current in the inductor, to operate the valves in a manner that prevents shorting of the power line and provides a conductive path for current in the inductor.
Preferably, the system is connected to the power line via a filter.
According to the present invention there is provided a method of supplying power from an AC power line to a load, including the steps of: (a) connecting a series valve in series with the load; (b) connecting a shunt valve in parallel with the load; (c) connecting an inductor in series with the load; (d) while the inductor carries current in phase with the power line, configuring the valves to operate as a buck converter; and (e) while the inductor carries current out of phase with the power line, configuring the valves to operate as a boost converter.
The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein:
The present invention is of a HF AC chopper which can be used to control delivery of power to a load. Specifically, the present invention can be used to provide an AC voltage that is less than the powerline voltage and is of substantially sinusoidal waveform to a load. The load can be resistive or reactive, and the current drawn from the power line is substantially sinusoidal and has minimal harmonic content.
The principles and operation of a HF AC chopper according to the present invention may be better understood with reference to the drawings and the accompanying description.
Referring now to the drawings,
When load 20 has a reactive impedance, current through output terminals 7 and 8, and therefore current I0 through inductor L, is of different phase with respect to output voltage V0. For example, in a case of a linear inductive load 20, current I0 lags voltage V0 as shown schematically in
To accommodate all possible combinations of voltage and current polarity, and hence, power flow in either direction, to avoid the need for a deadtime during which inductor current I0 is interrupted, and to avoid a short circuit across input terminals 1 and 2, the present invention utilizes diode-switch assemblies to provide safe current paths at all times. This is accomplished by first, always providing a current path via a diode or a switch, and second, blocking the possibility of a short circuit across input terminals 1 and 2 by placing diodes in series with switches such that either the diode or the switch conducts. Accordingly, each diode-switch assembly includes diodes and switches that are activated during specific time intervals. The operation of these devices according to the present invention is further detailed below with reference to
Preferably, a diode incorporated in a diode-switch assembly of a HF AC chopper according to the present invention has a short forward recovery time to prevent excessive voltage across any valve or valves that are in substantially non-conductive states at times when the diode is beginning to conduct.
According to the present invention, during T3, which is similar to T1, except that the directions of the voltages and currents are reversed, series diode-switch assembly 3 is configured to act as an active chopper switch, S3, and shunt diode-switch assembly 4 is configured to serve as a freewheeling diode, D3, as illustrated schematically in
Time interval T4 is similar to T2 in that power flows back to the source. In this case, illustrated schematically in
The operation illustrated schematically in
According to the present invention, switch operation is not limited to the sequence shown in
As will be readily apparent to those skilled in the art, generation of switch control signals according to the present invention is easily accomplished by use of devices including, but not limited to, dedicated logic circuitry, programmable logic circuitry including, but not limited to, a Field Programmable Gate Array (FPGA), or a microprocessor. Furthermore, as will be readily apparent to those skilled in the art, the transition from one diode-switch operational regime to another is done while fulfilling the above-stated objectives, i.e., keeping at all times an open path for the inductor current and avoiding a short circuit across input terminals 1 and 2. This is illustrated schematically in
It will be readily apparent to those skilled in the art that the chopping frequency need not necessarily be constant, and that the duty cycle ratio also need not necessarily be constant within any particular interval, and that allowing the chopping frequency and duty cycle ratio to vary in a controlled fashion provides opportunities to control the output of a HF AC chopper according to the present invention in a flexible manner. Several variations of the present invention that take advantage of this flexibility are described below, and such variations, including, but not limited to those described herein, are within the scope of the present invention.
Another possible embodiment of a HF AC chopper according to the present invention is illustrated schematically in
An important feature of a HF AC chopper according to the present invention is the ability to protect the circuitry against high current, as might develop if the output is shorted, or as a result of inrush current. Inrush current develops in various loads in response to a fast increase in output voltage. For example, in incandescent lamp dimmer applications, inrush current develops when the lamp filaments are cold until the filaments reach the elevated operating temperature. This is because of the large difference in the resistance of a filament when the filament is cold and when the filament is hot. Under typical operating conditions, the resistance of a hot filament is typically more than 10 times the resistance of a cold filament. Consequently, when a voltage is first fed to an incandescent lamp, the current is much greater than the nominal operating current. This might cause interference on the power line feeding the lamp and could harm the HF AC chopper. Similarly, a short circuit causes high currents to build up and, unless controlled, damages the HF AC chopper. In the case of high current buildup, a deadtime could be especially harmful because interruption of the high current generates very large voltage spikes that can easily cause breakdown. The present invention solves the protection problem by first limiting the duty cycle ratio to lower the output voltage and, if required, bringing the duty cycle ratio down to zero, while always providing a conduction path for the inductor current I0. This is accomplished inexpensively via a software routine in the case of a microcontroller-based system, or by additional logic circuitry in the case of an analog-digital implementation.
The current is limited according to a limit profile, which allows flexibility in the limitation of overcurrents. In one class of embodiments of the present invention, such a profile allows large overcurrents for very short periods, moderate overcurrents for longer periods, and small overcurrents for even longer periods, in a manner similar to a “slow-blow” fuse or circuit breaker. In the simplest case, a limit profile includes a single overcurrent setting. All such limit profiles are within the scope of the present invention.
In another mode of operation according to the present invention, an output feedback path is used to control the waveform of output voltage V0 such that output voltage V0 has a waveform different from the waveform of input voltage Vin, provided that the target for output voltage V0, at any given time, is lower than the input voltage Vin at that time. In this case control signal Vc is not used only to designate an attenuation level, but rather as a reference that varies with time. For example, Vc can be a pure sinusoidal reference voltage and the feedback arrangement forces output voltage V0 to follow that reference voltage even if input voltage Vin has a distorted waveform. Similarly, it is possible to use output current I0 as a feedback variable, in which case the HF AC chopper acts as a current source, which is desirable for loads for which it is preferable to control current, rather than voltage. Such loads include, but are not limited to, discharge lamps, light emitting diodes, lasers, and arc welders.
Optionally, an additional current sensor 16 is used to tightly control the input current Iin so as to either follow input voltage Vin, or to have of a pure sinusoidal waveform. This is important in cases where the load is non-linear and, hence, causes input current Iin to be distorted. This is remedied, according to the present invention, by sensing input current Iin via a sensor 15 operative to produce on line 16 a signal proportional to input current Iin, and closing a feedback loop such as to force input current Iin in to follow the desired waveform while, at the same time, keeping the output at the desired power level. This arrangement for controlling the waveform of input current Iin as well as controlling the output power level is well known in the art and is widely used in so-called Active Power Factor Correction (APFC) power stages.
Diode-switch assemblies are not restricted to one particular topology. For example, in
While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.
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|Nov 17, 2004||AS||Assignment|
Owner name: COMPULITE SYSTEMS (2000) LTD., ISRAEL
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:BEN-YAACOV, SHMUEL;REEL/FRAME:015997/0221
Effective date: 20041114